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08-03-2011, 04:24 PM   #1
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FOCAL REDUCER? Questions for astrophotographers

So I'm slowly researching an article about stuff to hang onto camera lenses, and I come across references to FOCAL REDUCERS. Apparently these are well-know in the astro realm. Maybe I'm just not spaced enough... Anyway, I find little info about their significance and application. Perhaps some astrophotographers here can tell us about FR's. Here is what I think I know now, and what I'd like to know:

A Focal Reducer is a back-of-the-lens add-on like a TeleConverter, but with just the opposite effect. A TC increases focal length and decreases effective aperture; an FR *decreases* focal length and *increases* apparent aperture. So a 0.5x FR mounted on a 1000mm f/10 telescope turns it into a 500mm f/5 'scope -- a wider visual field, and it lets in more light. And this raises questions.

* What are the limitations? Is there a minimum focal length to which an FR can be applied? Can I mount my 24/2 on one to make it a 12/1?

* What are the implications? Is the camera-lens register (flange focal distance) affected? Is distortion introduced? Is IQ lowered noticeably? (The picture's IQ, that is -- my IQ is low enough already.)

* What is the construction? Can I just take an ordinary 2x TC, flip the optical element over, and turn it into a FR? Or must I buy one?

* On eBay I see various 0.5x and 0.7x etc FR's in 1.25in and 2in sizes, costing US$20-100+. I can probably adapt a 2in body to an M42 or PK mount. Are any of the cheap offerings any good, or just the usual optical trash?

* Should those of us NOT doing astro shooting even bother thinking about FR's? Have I just opened a worthless can of worms?

Please enlighten us (and me), oh Masters of the Skies!

EDIT: Yes, I gargled for FOCAL REDUCER LENS and the first hit is this article. I'm just fatigued and I'm having trouble digesting it. Simplification is welcomed!

08-03-2011, 05:09 PM   #2
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The ST8 CCD the linked article says the reducer will illuminate is a little more than half the size of an pentax sensor.

ST-8 Information

But I really know nothing about this stuff
08-03-2011, 05:13 PM   #3
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QuoteOriginally posted by newarts Quote
The ST8 CCD the linked article says the reducer will illuminate is a little more than half the size of an pentax sensor.
That article is also about 12 years old! So I'd trust it for an explanation of general principles, not current product details. Newer products hopefully cover larger frames.
08-03-2011, 05:50 PM   #4
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I am going to talk to the guy that owns this place,

Spencers Camera & Photo Digital Camera Infrared (IR) Conversions, Modifications and IR Photography :: Welcome

tomorrow about a lens repair, You could always email him on your own if you want to. If he says anything interesting I will post it.

08-03-2011, 08:33 PM   #5
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QuoteOriginally posted by RioRico Quote
(lots of text snipped) I'm just fatigued and I'm having trouble digesting it. Simplification is welcomed!
At the risk of oversimplification, think of a focal reducer like a reverse teleconverter. It reduces the focal length of a telescope and also increases the aperture (decreases the F-stop number). For example, a 2000mm F10 telescope with a 0.5 reducer functions as a 1000mm F5 telescope.

The 1.25" and 2" reducers you've seen for sale probably screw onto a telescope eyepiece. Some other focal reducers screw onto the rear cell of the telescope and are designed for specific types of telescopes. A poor telescope and reducer pairing causes vignetting and makes it impossible to reach focus due to flange distance. The more reduction you have the more likely these problems will show up.

Purchasing a telescope reducer in hopes of adapting it for camera lenses might be a productive exercise, but I expect it's likely to be more of a learning experience that cannot turn a 24/2 camera lens into 12/1.
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